1. Field of the Invention
The present invention relates generally to audio processing, and more particularly to techniques for active noise cancellation.
2. Description of Related Art
An active noise cancellation (ANC) system in an earpiece-based audio device can be used to reduce background noise. The ANC system forms a compensation signal adapted to cancel background noise at a listening position inside the earpiece. The compensation signal is provided to an audio transducer (e.g. a loudspeaker) which generates an “anti-noise” acoustic wave. The anti-noise acoustic wave is intended to attenuate or eliminate the background noise at the listening position via destructive interference, so that only the desired audio remains. Consequently, the combination of the anti-noise acoustic wave and the background noise at the listening position results in cancellation of both and hence a reduction in noise.
ANC systems may generally be divided into feedforward ANC systems and feedback ANC systems. In a typical feedforward ANC system, a reference microphone provides a reference signal based on the background noise captured at a reference position. The reference signal is then used by the ANC system to predict the background noise at the listening position so that it can be cancelled. Typically, this prediction utilizes a transfer function which models the acoustic path from the reference position to the listening position. Active noise cancellation is then performed to form a compensation signal adapted to cancel the noise, whereby the reference signal is inverted, weighted and delayed or more generally filtered based on the transfer function.
Errors in feedforward active noise cancellation can occur due to the difficultly in forming a transfer function which accurately models the acoustic path from the reference position to the listening position. Specifically, since the surrounding acoustic environment is rarely fixed, the background noise at the listening position is constantly changing. For example, the location and number of noise sources which form the resultant background noise can change over time. These changes affect the acoustic path from the reference position to the listening position. For example, the propagation delay of the background noise between the reference position and the listening position depends on the direction (or directions) the background noise is coming from. Similarly, the amplitude difference of the background noise at the reference position and at the listening position may be direction dependent.
In a feedforward ANC system, an error microphone may also be placed within the earpiece to provide an error signal. The error signal is intended to indicate the residual noise that remains within the earpiece after cancellation has taken place. The ANC system can then adjust the parameters of the transfer function in an attempt to drive the error signal to zero. However, since the error microphone also captures the desired audio as output by the audio transducer, it can be difficult to accurately determine the residual noise. In addition, it can be difficult to avoid unwanted direct feedback of the desired audio through the reference microphone, which can lead to stability problems for the ANC system.
If the transfer function used to model the acoustic path from the reference position to the listening position is even slightly incorrect, residual noise will remain at the listening position. The residual noise can interfere with the listening experience of the desired sound, and is annoying. In some instances, errors in the transfer function may result in the generation of an anti-noise acoustic wave that constructively interferes with the background noise at the listening position. In such a case, the combination of the anti-noise acoustic wave and the background noise may result in an increase in the noise at the listening position, rather than a decrease.
It is therefore desirable to provide active noise cancellation techniques which can increase the quality and robustness of active noise cancellation systems in diverse acoustic environments.